Particular applications for the constituent components of air often require that components be produced as liquid products from the air separation plant. Elevated pressure cryogenic air separation cycles have the advantages of smaller equipment size and smaller diameter pipelines, as well as energy loss due to pressure drops across these pipelines and equipment. Unfortunately, nitrogen produced by an elevated pressure air separation plant is typically at a higher pressure than is required for its use. The energy of this surplus pressure of the nitrogen from an elevated pressure cycle can be utilized to produce liquid products. With the availability of this excess pressure energy the quest is to find more efficient ways of utilizing the pressure energy of the nitrogen product from elevated pressure cycles.
The conventional way of making liquid oxygen and/or liquid nitrogen is to add a liquefier to the low pressure cycle air separation unit in which the low pressure column operates in the pressure range of about 2-9 psig. The liquefier may be integrated into the air separation plants, such as is shown in U.S. Pat. No. 4,152,130 in which compressed air is expanded to provide the refrigeration needed for liquefaction. Air expansion cycles have the disadvantage that if large quantities of liquid nitrogen product are required, then argon and oxygen recoveries will severely suffer.
U.S. Pat. No. 4,705,548 teaches the use of heat pumping with nitrogen to help solve this recovery problem, but, unfortunately, this heat pumping step introduces inefficiencies by increasing exergy loss in heat exchangers and increases capital cost.
Great Britain Pat. No. 1,450,164 suggests increasing the operational pressure of the air separation unit thereby producing an increased pressure nitrogen product and then using this pressure energy to supplement the refrigeration needed for the production of liquid oxygen. This cycle is not efficient because of the unnecessary degree of energy degradation in utilizing the refrigeration produced by expansion of the pressurized nitrogen.
Another problem of conventional air separation plants is that typically large amounts of waste nitrogen are used for producing chilled water, which needs to be at a pressure very close to atmospheric pressure (e.g. about 0.5 psi higher than atmospheric pressure), and for regeneration of the mole sieve beds, which needs to be at a pressure 1-3 psi higher than atmospheric pressure. Conventionally, both streams are produced from the low pressure column, with the pressure of the low pressure column being set by the pressure of the mole sieve regeneration stream, resulting in a higher column pressure and therefore a higher discharge pressure from the main air compressor. The other way to set the pressure of the low pressure column is according to the water chilling nitrogen stream pressure and compress the regeneration stream to the required pressure. This solution requires more capital since the regeneration stream pressure booster and after-cooler adds to the capital cost.